How can I fix the streaking on the side of this cube in Blender 2.83?
It appears as though either: 1. you are not using the UVmap you created, 2. there a different materials applied to different faces, or 3. The object is not unwrapped correctly.
First, make sure that in your node setup you have the UV output on the Texture Coordinate node connected to vector input socket on Mapping node.
Second, make sure that all your faces have the same material assigned. Select all of your faces in edit mode and in the materials tab, choose the correct material and click assign.
Third, try unwrapping again. Go into edit mode. select all of your faces. choose UV Unwrap.